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WorksheetsGRAVITY METHOD
Total questions: 25
Worksheet time: 16mins
What is observed gravity?
Gravity readings taken on site
Gravity readings after tide and drift correction
Gravity readings after drift correction
Gravity readings after tide correction
Which of the following represent Bouguer correction?
0.0419ρh
0.419ρh
0.00419ρh
0.0419h
Normally the datum used for gravity surveys is sea level and gravity decrease (a) mGal for every meter above sea level
Which one of the following represent Bouguer anomaly?
gobs - gφ + ∆g fac - ∆g Bouguer + ∆g terrain
gobs + gφ + ∆g fac - ∆g Bouguer - ∆g terrain
gobs + gφ + ∆g fac - ∆g Bouguer + ∆g terrain
gobs + gφ + ∆g fac + ∆g Bouguer + ∆g terrain
When do we need to do Eötvös correction in gravity method survey?
If measurements are made on moving vehicles such as car, plane, or ship
If there is a moving vehicle nearby the gravity station
If your base station is moving
If you are moving
Which of the following includes in spatial variations in gravity corrections?
i. Bouguer correction
ii. Terrain correction
iii. Free-air correction
iv. Tidal correction
i, ii and iii
ii, iii and iv
i, iii and iv
i, ii, iii and iv
The main end product of gravity data correction is the Bouguer anomaly which should only correlate with lateral variations in density of the upper crust.
TRUE
FALSE
Which one of the materials has the highest density value?
Quartzite
Shale
Basalt
Pyrite
1 mGal is equal to?
1 × 10^5 ms^-2
1 ms^-2
1 × 10^-5 ms^-2
10 ms^-2
The following statements are true EXCEPT:
A gravimeter is any instrument designed to measure spatial variations in gravitational acceleration
Relative to areas near sea level, mountainous areas would have extra mass, tending to decrease the gravity value
Latitude variation is changes in the observed acceleration caused by the ellipsoidal shape and the rotation of the Earth
Any movement that caused the ground to vibrate will also affect the gravity reading
Which of the following statements are TRUE?
i. Tidal effects cannot be predicted accurately
ii. Latitude correction is a correction that accounts for the Earth’s elliptical shape and rotation
iii. Free air correction considers both elevation differences relative to a datum as well as the mass between the observations point and datum
iv. 2.67 g/cm^3 is chosen as the average density values because it gives the least influence of relief features
i and ii
ii and iii
ii and iv
iii and iv
What is the depth rule for sphere in gravity anomalies?
z = 1.0x1/2
z = 0.58x1/2
z = 1.305x1/2
z = 0.4x1/2
A broad, low frequency anomaly will generally indicate a _____________body.
deep
shallow
Gravity decreases as we go towards the equator, so in Bouguer anomaly calculations, we add ______________________ to it as we move to the equator.
Latitude correction
Free air correction
Bouguer correction
Terrain correction
A gravity anomaly is the difference between the ______________ gravity and the _______________ value of gravity at the station.
raw, predicted
true, modeled
corrected, predicted
observed, modeled
All of these are factors that can cause variations in acceleration that would be observed even if the gravimeter is not moved EXCEPT:
Slab effects
Instrument drift
Tidal effects
Which one of these statements is TRUE?
Nop setting up his gravimeter instrument to automatically remove the Earth tide effect and did not include tidal correction in data reduction process.
Harie doing looping technique in his gravity data acquisition to correct for latitude effect.
Anith subtracting free air correction from the observed gravity to correct for elevation effect.
Aida did a gravity data acquisition in a flat area and include terrain correction in her gravity data reduction process.
Which statements regarding gravity anomaly are TRUE?
i. The difference between what is measured and what is expected assuming the reading is made on non-homogenous spheroid.
ii. Always a sum of several components which are often difficult to isolate individually.
iii. The deeper below the Earth’s surface a structure affecting gravity is situated, the larger in amplitude and broader its contribution to the total anomaly is.
iv. One can calculate the gravity effect of any mass distribution but cannot unambiguously deduce mass distribution from measured gravity values
ii and iii
i, and iv
iii and iv
ii and iv
Which statements regarding densities of geologic materials are TRUE?
i. Density increases with decreasing porosity
ii. Density decreases with decreasing in grade of metamorphism
iii. Felsic rocks are less dense than mafic rocks
iv. Density decrease with low silica content
i, ii and iii
i, ii and iv
i, iii and iv
ii, iii and iv
Why do we need to analyze Bouguer anomalies?
i. To ensure that the signals comes from geological structures only
ii. Because deeper and larger sources may mask our sources of interest
iii. To determine regionals and residuals
iv. To eliminate all unwanted noises
i and iii
i and iv
iii and iv
ii and iii
The edges of the body will tend to lie under inflection points on the gravity profile.
TRUE
FALSE
Which one of these statements is TRUE?
Muiz is mapping a near-vertical fault so he chose geometrical form of horizontal cylinder for his geological model
Janani determine the depth of the geological structure from the Bouguer anomaly contour map
Zaim used the depth rule of z=x1/2 because the geological structure he is mapping is thin dikes.
To minimize the ambiguity in interpretations, Diyana collect samples from her study area, utilizing both gravity and magnetic methods for her study and collect all sorts of geological information available for her study area.
Maximum gravity occurs directly over the center of the sphere.
TRUE
FALSE
Gravity method is a _________ method that utilizes the naturally occurring gravitational field in the Earth.
active
passive
A sharp high frequency anomaly will generally indicate a deep body.
TRUE
FALSE
